2492 M. Schmaicker et al Journal of the European Ceramic Society 20(2000)2491-2497 mullite was employed(68 wt. Al2O3) though some before grinding and polishing. SEM was performed glassy phase had to be accepted in the resulting mullite using a LEO Gemini 982 microscope equipped with a matrix. The porous mullite matrix composite shows field quasi-ductile fracture behaviour and strength values of Al2 D3 /SiO ra I cathode and an Oxford EDX system. The atio of matrix mullite crystals was deter more than 300 MPa in case of unidirectional reinforce- mined via lattice constant data I using fibre-free model ment with a fibre content of 45 vol%.8, 9 Since the samples. Lattice constants were obtained from careful fibre strength after 1300C heat treatment is >1500 X-ray diffraction(XRD) measurements using a Siemens MPa, the maximum strength of the composite is sig- D 5000 XRD machine. In selected specimens, the com- nificantly smaller than one would expect using the rule position of the submicron-sized matrix mullite crystals of mixture. Therefore, the strength of the composite was checked by eDX in combination with a transmis- seems to be controlled by fibre/matrix debonding pro- sion electron microscope(Philips EM 430 equipped with cesses rather than by direct fibre strength. Although the a Tracor EDX system) matrix contains more than 5% glassy phase, there is no Three-point bending tests (40 mm span) were per excessive creep deformation of the composite at elevated formed on 50x 5xl mm bars cut out of the lD-compo temperatures. Preliminary investigations show that the site material. At least eight specimens were tested for reep behavior of the composites is controlled by the each firing series creep resistivity of the fibres rather than by matrix properties. Similar results recently were reported by Deng investigating SiC-fibre/mullite matrix compo- The aim of the present study is the investigation of thermally induced reactions between the silica-rich matrix and alumina-rich Nextel 720 fibres Reactions between SiO(in the matrix)and o-AlO3(in the fibres can be expected according to the Al2Ox-SiO phase diagram when the thermal activation of the samples is sufficient. The reactions between fibres and matrix have implications for the mechanical properties of the com posite, which will be discussed in detail 2. Experimental 2.1. Materials processing Fig. I. Overview of the as-prepared alumino silicate fibre/porous mullite matrix composite( scanning electron micrograph from polished Green bodies of the porous mullite matrix composite ere fabricated by infiltration of fibre bundles with an aqueous mullite precursor (Siral, Ce once Germany) slurry and subsequent winding up on a mandrel. The infiltrated fibre tapes were removed from the mandrel in the moist stage, rolled in flat tapes, and sintered pres useless in air at 1300 C( 60 min). The fibre content of the ID-composite is approx. 45 vol % Details of the CMC processing are published elsewhere. CMC sam- ples were heat-treated at 1300 C(1000 h)and at 1400, 1500 and 1600 C(2 h)in air. 2.2. Characterization The microstructural development was monitored by means of scanning electron microscopy (SEM)on polished sections. Due to the high matrix porosity, the samples were infiltrated with a low-viscous epoxy resin Fig. 2. Detail of the as-prepared alumino silicate fibre/porous mullite f The composition of 3M Nextel 720 fibre is 85 wt %Al2O3,I matrix composite. Note the high-porous mullite matrix with small wt% SiOz. The fibre consists of a-Al2O3 plus mullite glassy pockets existing between the rectangular mullite crystalsmullite was employed (68 wt.% Al2O3) though some glassy phase had to be accepted in the resulting mullite matrix. The porous mullite matrix composite shows quasi-ductile fracture behaviour and strength values of more than 300 MPa in case of unidirectional reinforce￾ment with a ®bre content of 45 vol.%.8,9 Since the ®bre strength after 1300C heat treatment is >1500 MPa,17 the maximum strength of the composite is sig￾ni®cantly smaller than one would expect using the rule of mixture. Therefore, the strength of the composite seems to be controlled by ®bre/matrix debonding pro￾cesses rather than by direct ®bre strength. Although the matrix contains more than 5% glassy phase, there is no excessive creep deformation of the composite at elevated temperatures. Preliminary investigations show that the creep behavior of the composites is controlled by the creep resistivity of the ®bres rather than by matrix properties. Similar results recently were reported by Deng investigating SiC-®bre/mullite matrix compo￾sites.10 The aim of the present study is the investigation of thermally induced reactions between the silica-rich matrix and alumina-rich Nextel 720 ®bres.y Reactions between SiO2 (in the matrix) and a-Al2O3 (in the ®bres) can be expected according to the Al2O3±SiO2 phase diagram when the thermal activation of the samples is sucient. The reactions between ®bres and matrix have implications for the mechanical properties of the com￾posite, which will be discussed in detail. 2. Experimental 2.1. Materials processing Green bodies of the porous mullite matrix composites were fabricated by in®ltration of ®bre bundles with an aqueous mullite precursor (Siral, Condea, Germany) slurry and subsequent winding up on a mandrel. The in®ltrated ®bre tapes were removed from the mandrel in the moist stage, rolled in ¯at tapes, and sintered pres￾sureless in air at 1300C (60 min). The ®bre content of the 1D-composite is approx. 45 vol.%. Details of the CMC processing are published elsewhere.8 CMC sam￾ples were heat-treated at 1300C (1000 h) and at 1400, 1500 and 1600C (2 h) in air. 2.2. Characterization The microstructural development was monitored by means of scanning electron microscopy (SEM) on polished sections. Due to the high matrix porosity, the samples were in®ltrated with a low-viscous epoxy resin before grinding and polishing. SEM was performed using a LEO Gemini 982 microscope equipped with a ®eld emission cathode and an Oxford EDX system. The Al2O3/SiO2 ratio of matrix mullite crystals was deter￾mined via lattice constant data11 using ®bre-free model samples. Lattice constants were obtained from careful X-ray di€raction (XRD) measurements using a Siemens D 5000 XRD machine. In selected specimens, the com￾position of the submicron-sized matrix mullite crystals was checked by EDX in combination with a transmis￾sion electron microscope (Philips EM 430 equipped with a Tracor EDX system). Three-point bending tests (40 mm span) were per￾formed on 5051 mm bars cut out of the 1D-compo￾site material. At least eight specimens were tested for each ®ring series. Fig. 1. Overview of the as-prepared alumino silicate ®bre/porous mullite matrix composite (scanning electron micrograph from polished cross-section). Fig. 2. Detail of the as-prepared alumino silicate ®bre/porous mullite matrix composite. Note the high-porous mullite matrix with small glassy pockets existing between the rectangular mullite crystals. y The composition of 3M Nextel 720 ®bre is 85 wt.% Al2O3, 15 wt.% SiO2. The ®bre consists of a-Al2O3 plus mullite. 2492 M. SchmuÈcker et al. / Journal of the European Ceramic Society 20 (2000) 2491±2497